To avoid short-circuiting in transformer windings, it is a requirement that they should at all times, and independently of the temperature of the transformer, be prestressed with a certain compressive stress. The plastic settling of the insulating material, however, leads to a gradual decrease of the clamping force. Because the insulating material has a considerably greater coefficient of thermal expansion than the core material, a thermal increase of pressure takes place with rising temperature and a corresponding thermal decrease of pressure with decreasing temperature. At low temperatures the prestress may be zero.